f150 won't start on just 1 battery

[panasonic]

"Next pics are voltage drop results , starting with the positive side key off , then on , then following with negative side in the same order."



Just to be sure, you said with the key "ON", meaning you where cranking the engine over while doing the voltage drop on each cable?

Also this is how you had your meter hooked up? (see attached diagram)

[gsdanno1]

Yes on the meter hookup , no on the cranking, key was turned to the on position

[pstephens46]

I think that 9.1 reading is telling. As long as the you are reading pos to pos. That should be an indication of resistance.

I would check any connections on that wire run. Back your lead up and take another measurement.

[gsdanno1]

There aren't any connections on that run. It goes from the starter straight to the battery. From th et e to the switch . Everything cleaned and tightened again this morning

[gsdanno1]

Going positive to positive from batt to batt switch I get -53 mv

[gsdanno1]

Now voltage drop is -39mv key off. And -119 mv. How can I be reading a negative number

[pstephens46]

Now voltage drop is -39mv key off. And -119 mv. How can I be reading a negative number

Reverse polarity. Try reversing leads .

I cant see the pics very well on my phone. Was that 9.1 mv or 9.1volts in third pic above?

[gsdanno1]

9.1 mv , but just now I have 000.0mv key off , and 5.9 mv key off neg to neg ,and 163mv key on positive to positive. The more I check the more confused I become. Can't understand how there could be a polarity issue

[pstephens46]

Me either. Maybe Rodbolt can weigh in.

[99yam40]

the .5 volt Rod talked about is 500 MV
you are below that, so go on to the next part of test.

10 seconds of cranking is a long time on a starter.

[gsdanno1]

Would love to , what would the next test be

[pstephens46]

Did you have the battery switch set to both while testing?

Perhaps you need to check voltage drop to the switch while switch is turned to 1 and then 2. Issue may be in the "short" run of the cable there between battery and switch. Would explain why it would start when set on both versus 1 or 2.

[panasonic]

Yes on the meter hookup , no on the cranking, key was turned to the on position

Do the same exact test on the positve and negative cables again while cranking the engine over. See what the meter reads then....should be no more then .5 to .7 volts DC. (Which is 500 to 700 millivolts) Tell us what you find.

[panasonic]

Reading back through the replies I don't think it was clear that you needed to be cranking the engine over with the meter hooked up like you had it. You are trying to see if there is a voltage drop in one or both of the starter cables WHEN they are under load. You do it FIRST with no load to get a baseline reading, then do it again while cranking the engine to put voltage pressure through the cable. I hope that helps you out.

[gsdanno1]

I did ,readings were all over the place , I'll try again and see if I can get something steady enough to write down